The overall goal is to understand the role of cytochrome P-450 in the regulation of physiologic concentrations of steroid hormones in the body. The objectives of the proposed research are to investigate 1. The role of phospholipids and steroids in the regulation of electron transport to P-450 in the steroid C-21 hydroxylase system. 2. The possible involvement of phospholipids in the interaction of steroids with cytochrome P-450. The proposed experiments will be performed using the following types of enzyme preparations: 1. Bovine adrenocortical microsomes, 2. Phospholipid-depleted microsomes. 3. The hydroxylase system resolved into P-450-containing particulate fraction and the soluble reductase fraction. The proposed research includes the study of -- 1. The influence of phospholipids and steroids on P-450 reduction in the recombined fractions (kinetic studies) 2. The influence of phospholipids on the interaction of steroids with cytochrome P-450 (includes difference spectrophotometry) 3. The influence of phospholipids and steroid substrate on the rate of hydroxylated product formation (radio-isotope tracer technique).